1. Field of the Invention
The present invention generally relates to thermal mass flow sensors on measuring the flow rate of flowing gas or liquid, more particularly; it concerns mass flow sensors which are made of Micro Electro Mechanical Systems (MEMS) approach and methods of manufacture. The present MEMS flow sensor is built on an active membrane or a porous silicon based device.
The active membrane or the porous silicon device is capable of generating a self-cleaning surface wave, such as ultrasonic acoustic wave emitted from porous silicon, on the active region of sensor surface. With the self-cleaning capability at the time of desire, the sensor itself would be able to work at environments with alien particles or debris from its active region. The sustained cleanness of active area on sensor surface could greatly enhance the flow measuring accuracy and retain its repeatability from calibration.
2. Description of the Related Art
Various micromachined gas and liquid mass flow sensors have been heretofore developed and commercially available on the market. One of the popular mass flow sensors, thermal mass flow sensor, is operated based on the principle of hot-wire anemometry or calorimetry. Compared to MEMS technology, the conventional technologies of mass flow sensors are constrained by the low flow uncertainties, small turn-down ratio on flow rate measurement, and high power consumption. In particular, the conventional thermal mass flow meters often become malfunction in a dirty fluid channel such as the fluid flow with smoke or dust in an electrical industrial environment. The malfunction of the meters lead to interruption of the manufacture and the sensors after liquid or dry cleaning may not have the same performance besides the installation deviations.
The US patents (Philip J. Bohrer and Robert G. Johnson, Flow Sensor, U.S. Pat. No. 4,478,077; Robert E. Higashi, Semiconductor device microstructure, U.S. Pat. No. 4,696,188) teach a typical MEMS type of thermal mass flow sensor generally comprised of a micro-machined membrane which is functioned as thermal isolation purpose to improve the accuracy of device operation. Therein the active region of membrane usually consists of heating elements and sensing elements such as resistance temperature detector (RTD) or thermopiles. However, this type of MEMS thermal flow sensor would have the following limitations:                (1) The thin film membrane is very fragile to be damaged and cause the device to malfunction frequently in a dusted or smoked fluid.        (2) The opening slots on the surface of membrane are constructed to block the heat conduction horizontally. However the open slots will also easily trapping the particles that may lead to the malfunction of the sensor.        (3) The opening slots on membrane will limit the application of flow sensor on liquid measurement because the filled liquid underneath the membrane will reduce the thermal resistance between membrane and the substrate to cause uncertainties.        
The U.S. Pat. No. 7,040,160 (Flow Sensor; by Hans Artmann et al.) teaches a thermal flow sensor built on a region having poor heat conductivity of a silicon substrate. The region having poor heat conductivity is made of porous silicon or porous silicon dioxide. However, in the embodiment of this patent, the lateral thermal conductivity between the heater element and sensor components, unlike the current invention, is not totally isolated. The lateral heat conduction through the cover layer in Hans Artmann's invention can significantly reduce the measuring accuracy. Moreover, the location of the ambient temperature sensor to detect the environmental temperature for heater temperature control is either omitted or not specified explicitly in Hans Artmann's invention. To ensure the ambient temperature sensor having good thermal conductivity to the substrate is very crucial to prevent the temperature effect of thermal flow sensor. For the above reason, the ambient temperature sensor should not be disposed on the porous silicon region. A further limitation of the Artmann's approach is that the sensor would not work in a dust or dirty flow fluid as the surface of the sensor would cover by such to cause the malfunction.
It would be desirable, therefore, to provide an apparatus and method whereby the mass flow meter could work robustly in a dirty flow fluid without being disrupted for improved work efficiency. To this end, it would also be desirable that the sensors can perform self-cleaning periodically to maintain the flow meter accuracy and productivity. It is further desirable to have the sensing element of the sensors being well isolated, and ensure the environment temperature detection sensor well functioned.
Self-cleaning usually involved special surface coating (U.S. Pat. No. 4,147,845, Article having self-cleaning coating, by A. Nishino et al., and U.S. Pat. No. 6,858,284, Surface rendered self-cleaning by hydrophobic structures, and process for their production, by E. Nun et al.). However, in a dirty or smoked fluid flow, modification of the surface structure often can only improve but not prevent from surface sticking of foreign materials. It is known that surface wave would be much stronger and active than merely passive surface coating in cleaning of the surface. But most of the cleaning apparatus involved a complicated system (U.S. Pat. No. 4,007,465, System for self-cleaning ink jet head, K. C. Chaudhary), and may not a simple adaptation for the said flow sensors.
It would be desirable, nonetheless, that a simple flow sensor with a high-performance in flow measurement yet a self-cleaned surface for reliability can be constructed. It is further desirable that the sensors shall be easily manufactured for a mass production. To this end, it is known that porous silicon can emit acoustic wave with strength (Characteristics of thermally induced ultrasonic emission from nanocrystalline porous silicon device under impulse operation, by Y. Watabe et al., Jap. J. Appl. Phys. Vol. 45, 3645-47). Alternative approach of the surface wave can be realized using an active capacitive force generated surface vibration. Such a structure can be simply constructed (U.S. Pat. No. 6,781,735, Fabry-Perot cavity manufactured with bulk micro-machining process applied on supporting substrate, Huang, et al.) Likewise, it is desirable to construct the mass flow sensor with self-cleaned surface by combining the mass flow sensor apparatus and surface acoustic emission or the surface vibration configuration.